1. Field of the Invention
This invention relates generally to image forming apparatuses. More particularly, this invention relates to an image forming apparatus which is applicable, for example, to copiers, laser printers and the like, wherein process members, such as a photosensitive member and a developing roller, are accommodated within a cartridge that is to be dismountably mounted on an apparatus main body.
2. Description of the Prior Art
There is a conventional image forming apparatus of this kind illustrated as one example in FIGS. 6 and 7. A process cartridge 1 includes a photosensitive drum 2 and a developing roller 3 which are in rotatable contact with each other with a center distance L kept constant therebetween. In an image forming apparatus of so-called a contact-development type having a developing roller 3 in contact with a photosensitive drum 2, the contact pressure exerting between the photosensitive drum 2 and the developing roller 3 has a great effect upon quality of images. In order to avoid this, the center distance L is fixedly kept to thereby prevent the contact pressure exerting between these process members from varying. The process cartridge 1 with such a structure is dismountably mounted on an apparatus main body 4 as shown in FIG. 7. When the process cartridge 1 is mounted onto the main body 4, a photosensitive member gear 5 provided in association with the photosensitive drum 2 to transmit a drive source of a not-shown drive motor to the photosensitive drum 2 is brought into engagement with a drive gear 6 provided on the apparatus main body 4, thereby delivering rotation to the photosensitive drum 2. Similarly, a developing roller gear 7 provided in association with the developing roller 3 to transmit a drive force to the developing roller 3 is also brought into engagement with the drive gear 8 provided on the apparatus main body 4, transmitting rotation to the developing roller 3.
However, the both process members are forcibly held in a fixed manner to maintain constant a center distance L between the photosensitive drum 2 and the developing roller 3. Accordingly, where variation is encountered in outside dimensions of the process members or in holes for positioning the process members, the center distance L cannot be kept constant, thus incurring a problem of degradation in images.
In order to cope with this, the conventional structure has been designed to absorb variation in center distance L by using an extremely soft material for a developing roller 3 or covering the developing roller 3 by an elastic film. Also, there is a developing apparatus disclosed in Japanese Utility Model Laying-Open No. S61-203756 G03G 15/08, 15/00! laid open on Dec. 22, 1986. In this prior art, the variation of contact pressure due to variation in drive force is prevented by providing a rotatable developing unit with a support point located close to a common tangential line on a photosensitive member and a developing roller and further providing a drive gear on a straight line passing through centers of the photosensitive member and the developing roller.
In the former structure, however, there involves many limitations to designing a material of the developing roller crucial to determine image quality and its peripheral arrangements. This raises a great obstruction in pursuing improvement of image quality and resolution.
In the latter structure, on the other hand, when a drive force is acted on by the drive gear in a direction away from a rotation axis of the photosensitive member, the developing roller moves away from the photosensitive member. Conversely, when a drive force is acted by the drive gear in a direction toward the rotation axis of the photosensitive member, there is a fear that the photosensitive member be urged to an excessive extent by the developing roller. That is, in the above prior art having the drive gear arranged on the straight line passing through the centers of the photosensitive member and the developing roller, a force is applied in such a direction that the developing roller is moved away from the photosensitive member (or in a direction of depressing photosensitive member by the developing roller). Thus, there has been a difficulty in maintaining constant the contact pressure that exerts between the photosensitive member and the developing roller.